Ultra-high-strength spring steel

ABSTRACT

An ultra-high-strength spring steel, for use as a valve spring steel in a vehicle engine, includes 0.5 to 0.7% by weight of C, 1.2 to 1.5% by weight of Si, 0.6 to 1.2% by weight of Mn, 0.6 to 1.2% by weight of Cr, 0.1 to 0.5% by weight of Mo, 0.05 to 0.8% by weight of Ni, 0.05 to 0.5% by weight of V, 0.05 to 0.5% by weight of Nb, 0.05 to 0.3% by weight of Ti, 0.3% or less by weight of Cu (but not 0%), 0.0001 to 0.3% by weight of Al, 0.03% or less by weight of N (but not 0%), 0.0001 to 0.003% by weight of O, and a remainder of Fe and other unavoidable impurities, based on 100% by weight of the ultrahigh-strength spring steel.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of priority to Korean Patent Application No. 10-2015-0171889, filed on Dec. 4, 2015 with the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to an ultra-high-strength spring steel, and more particularly to an ultra-high-strength spring steel having enhanced tensile strength and fatigue life for use as an engine valve spring.

BACKGROUND

Considerable attention is being directed to increasing vehicle fuel efficiency. To this end, vehicle weight reductions or a minimization of power loss by reducing friction is important. In addition, output efficiency maximization through increases of dynamic characteristics in the controlling of combustion of an engine itself is also important. Further, attempts to increase fuel efficiency by reducing dynamic loading through weight reductions of components performing dynamic behaviors in an engine head part have been made.

As engine valve springs, which are among components performing dynamic behaviors, directly control dynamic loading, a high fuel efficiency increase effect may be observed when their weight is reduced. As conventional valve spring materials, CrSi steel having a tensile strength of about 1900 MPa and CrSiV steel having a tensile strength of about 2100 MPa are commonly used. There have been attempts to develop high-strength spring steel having a tensile strength of 2100 MPa or more through the addition of alloy elements to the conventional CrSiV steel.

The above disclosed background art has been provided to aid in understanding of the present disclosure and should not be interpreted as conventional technology known to a person having ordinary skill in the art.

SUMMARY OF THE DISCLOSURE

The present disclosure has been made in view of the above problems, and it is an object of the present disclosure to provide ultra-high-strength spring steel having superior tensile strength to existing spring steels through optimization of the contents of Mo, Ni, V, Nb, and Ti, and superior fatigue strength through control of inclusions that enhance fatigue life.

In accordance with an aspect of the present disclosure, the above and other objects can be accomplished by the provision of an ultra-high-strength spring steel for use as a valve spring steel in a vehicle engine, including 0.5 to 0.7% by weight of C, 1.2 to 1.5% by weight of Si, 0.6 to 1.2% by weight of Mn, 0.6 to 1.2% by weight of Cr, 0.1 to 0.5% by weight of Mo, 0.05 to 0.8% by weight of Ni, 0.05 to 0.5% by weight of V, 0.05 to 0.5% by weight of Nb, 0.05 to 0.3% by weight of Ti, 0.3% or less by weight of Cu (but not 0%), 0.0001 to 0.3% by weight of Al, 0.03% or less by weight of N (but not 0%), 0.0001 to 0.003% by weight of O, and a remainder of Fe and other unavoidable impurities, based on 100% by weight of the ultra-high-strength spring steel.

The spring steel may have a tensile strength of 2300 MPa or more.

The spring steel may have fatigue strength of 1100 MPa or more.

The spring steel may have a yield strength of 2800 MPa or more.

The spring steel may have a hardness of 710 HV or more.

The sizes of inclusions present in the spring steel may be 15 μm or less.

In the inclusions, a fraction of inclusions having sizes of 10 to 15 μm may be 10% or less and a fraction of inclusions having sizes of less than 10 μm may be 90% or more.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and other advantages of the present disclosure will be more clearly understood from the following detailed description taken in conjunction with the accompanying drawings, in which:

FIG. 1 is a table representing ingredients of examples and comparative examples;

FIG. 2 is a table representing properties and performances of examples and comparative examples;

FIG. 3 illustrates graphs representing calculation results for phase transformation according to temperatures of ultra-high-strength spring steel according to an embodiment of the present disclosure; and

FIG. 4 illustrates graphs representing calculation results for phase transformation according to temperatures in cementite tissue of ultra-high-strength spring steel according to an embodiment of the present disclosure.

DETAILED DESCRIPTION

Reference will now be made in detail to the preferred embodiments of the present disclosure, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts.

FIG. 3 illustrates graphs representing calculation results for phase transformation according to a temperature of ultra-high-strength spring steel according to an embodiment of the present disclosure, and FIG. 4 illustrates graphs representing calculation results for phase transformation according to a temperature in cementite tissue of ultra-high-strength spring steel according to an embodiment of the present disclosure.

The ultra-high-strength spring steel according to the present disclosure may be valve spring steel used in a vehicle engine. In addition, the ultra-high-strength spring steel may be spring steel having enhanced tensile strength, fatigue strength, and the like according to optimization of main alloy ingredients. In particular, the ultra-high-strength spring steel may include 0.5 to 0.7% by weight of C, 1.2 to 1.5% by weight of Si, 0.6 to 1.2% by weight of Mn, 0.6 to 1.2% by weight of Cr, 0.1 to 0.5% by weight of Mo, 0.05 to 0.8% by weight of Ni, 0.05 to 0.5% by weight of V, 0.05 to 0.5% by weight of Nb, 0.05 to 0.3% by weight of Ti, 0.3% or less by weight of Cu (but not 0), 0.0001 to 0.3% by weight of Al, 0.03% or less by weight of N (but not 0), 0.0001 to 0.003% by weight of O, and a remainder of Fe and other unavoidable impurities, based on 100% by weight of the ultra-high-strength spring steel.

In the present disclosure, alloy ingredients and composition ranges thereof are limited for the following reasons. Hereinafter, “%” as a unit of the composition ranges indicates “% by weight”, unless specified otherwise.

The content of carbon (C) is preferably 0.5 to 0.7%. An increase of the carbon content in the steel provides a proportional increase in strength. When the content of carbon is less than 0.5%, strength increase is slight due to lack of hardenability during heat treatment. When the content of carbon is greater than 0.7%, martensite tissue is formed during hardening, and fatigue strength and toughness are decreased. Within the range, high strength and ductility may be secured.

The content of silicon (Si) is preferably 1.2 to 1.5%. Silicon increases elongation, heat resistance, and hardenability, and enhances permanent settability (shape preservation) by inhibiting shape change. In addition, silicon hardens ferrite and martensite tissues, and increases strength and resistance to tempering and softening when included in ferrite. When the content of silicon (Si) is less than 1.2%, resistance to tempering and softening is low. When the content of silicon (Si) is greater than 1.5%, heat resistance is increased, but the material becomes sensitive to decarbonization and decarbonization occurs during heat treatment.

The content of manganese (Mn) is preferably 0.6 to 1.2%. When manganese (Mn) as an element enhancing hardenability and strength is employed in a matrix, bending fatigue strength is enhanced and hardenability is increased. In addition, manganese (Mn) as a deoxidizer generating an oxide inhibits formation of inclusions such as Al₂O₃. When the content of manganese (Mn) is less than 0.6%, it is difficult to secure hardenability. When the content of manganese (Mn) is greater than 1.2%, toughness is decreased.

The content of chromium (Cr) is preferably 0.6 to 1.2%. Chromium, for securing toughness, may form a precipitate during tempering, enhance hardenability, increase strength by suppressing softening, and contribute to refinement of crystal grains and toughness increase. When the content of chromium (Cr) is 0.6% or more, superior tempering and softening characteristics, decarbonization, hardenability, and corrosion resistance are exhibited. When the content of chromium (Cr) is greater than 1.2%, intergranular carbides are excessively generated and strength decrease and brittleness may be caused.

The content of molybdenum (Mo) is preferably 0.1 to 0.5%. Like Cr, molybdenum forms a minute carbide precipitate, enhancing strength and fracture toughness. In particular, TiMoC having a size of 1 to 5 nm is uniformly formed to enhance tempering resistance and secure heat resistance and high strength. When the content of molybdenum (Mo) is less than 0.1%, it may be impossible to generate a carbide and strength is not sufficiently secured. When the content of molybdenum (Mo) is greater than 0.5%, precipitation and strength increase effects are saturated and thus it may be unnecessary to increase the content with respect to a cost aspect.

The content of nickel (Ni) is preferably 0.05 to 0.8%. Nickel, as an element helping to increase corrosion resistance, enhances heat resistance, prevents low-temperature brittleness and enhances hardenability, dimensional consistency and settability. When the content of nickel (Ni) is less than 0.05%, corrosion resistance and high-temperature stability are decreased. When the content of nickel (Ni) is greater than 0.8%, red brittleness may occur.

The content of vanadium (V) is preferably 0.05 to 0.5%. Vanadium as an element that enhances tissue refinement, tempering resistance, dimensional consistency, and settability, and secures heat resistance and high strength forms VC as a minute precipitate to increase fracture toughness. In particular, VC as a minute precipitate prevents movement of a grain boundary. In addition, V is dissolved and employed during austenitization, and precipitated during tempering, causing secondary hardening. When the content of V is less than 0.05%, prevention effect of fracture toughness decrease may be decreased. When content of vanadium (V) is greater than 0.5%, the size of the precipitate may be coarsened and, after quenching, hardness may be decreased.

The content of niobium (Nb) is preferably 0.05 to 0.5%. Niobium refines tissue, hardens a surface through nitrification, and enhances dimensional consistency and settability. In addition, strength may be increased through formation of NbC and generation rates of other carbides, such as CrC, VC, TiC, and MoC, are controlled. When the content of niobium (Nb) is less than 0.05%, strength may be decreased and the carbide may be heterogenized. When the content of niobium (Nb) is greater than 0.5%, generation of other carbides may be suppressed.

The content of titanium (Ti) is preferably 0.05 to 0.3%. Titanium prevents recrystallization of crystal grains such as Nb and Al and suppresses growth thereof. In addition, titanium forms nano-scale carbides such as TiC and TiMoC, reacts with nitrogen, suppresses crystal grain growth through generation of TiN, and minimizes hardenability decrease of BN by disturbing bonding of B to N through formation of TiB₂. When the content of titanium (Ti) is less than 0.05%, other inclusions such as Al₂O₃ are generated and thus fatigue durability is decreased. When the content of titanium (Ti) is greater than 0.3%, functions of other alloy elements may be disturbed and production costs may be increased.

The content of copper (Cu) is preferably 0.3% or less (but not 0). Copper increases quenching characteristics or strength after tempering, and, like Ni, increases corrosion resistance of steel. However, when the content of copper (Cu) is too high, alloy costs may be increased. Therefore, the content of copper (Cu) may be limited to 0.3% or less.

The content of aluminum (Al) is preferably 0.0001 to 0.3%. Aluminum reacts with nitrogen, refines austenite through formation of AlN, and increases strength and impact toughness. In particular, through an addition of Nb, Ti, and Mo, addition amounts of vanadium for refining crystal grains and nickel for securing toughness, as high-cost elements, may be reduced. When the content of aluminum (Al) is less than 0.0001%, effects due to addition of aluminum (Al) might not be anticipated. When the content of aluminum (Al) is greater than 0.3%, large square-shaped inclusions (Al₂O₃) are generated. Such large square-shaped inclusions may function as fatigue-stating points that decrease durability by weakening steel.

The content of nitrogen (N) is preferably 0.03% or less (but not 0). Nitrogen forms AlN and TiN through reaction with Al and Ti to exhibit crystal grain refinement effects and maximize hardenability of boron through formation of TiN. However, when the content of nitrogen (N) is too high, hardenability of steel may be weakened through reaction with boron. Therefore, the content of nitrogen (N) may be preferably limited to 0.03% or less.

The content of oxygen (O) is preferably 0.0001 to 0.003%. Since oxygen binds with Si or Al, forming hard oxide-based nonmetal inclusions and causing decrease of fatigue life characteristics, it is preferred to maintain the content of oxygen (O) as low as possible. However, with respect to a steel-making technology aspect, it is difficult to maintain the content of oxygen (O) at less than 0.0001%. Accordingly, in the present disclosure, the lowest content of oxygen (O) is 0.003%.

Meanwhile, remaining ingredients, except for the aforementioned ingredients, are Fe and other unavoidable impurities.

Hereinafter, the present disclosure will be described in more detail with reference to the following examples and comparative examples.

Experiments of producing spring steel according to each of examples and comparative examples were carried out under commercial spring steel production conditions. The content of each ingredient as summarized in FIG. 1 was changed to produce ingot steel. A wire rod made of the ingot steel was manufactured into a steel wire through sequential isothermal heat treatment, wire drawing, hardening-tempering, and hardening in a soldering bath. In particular, the wire rod was maintained at 940 to 960° C. for three to five minutes and then rapidly cooled at 640 to 660° C., followed by cooling to 18 to 22° C. for 0.5 to 1.5 minutes. Such isothermal heat treatment was performed to help a subsequent wire drawing process to be easily performed. Through this heat treatment, perlite was generated in the wire rod.

The wire rod subjected to the isothermal heat treatment was manufactured into a wire rod having a desired diameter through several wire drawing steps. In the present disclosure, wire-drawing was carried out such that a wire rod had a diameter of 3.3 mm.

The wire-drawn wire rod was re-heated and maintained at 940 to 960° C. for three to five minutes. Subsequently, rapid cooling was carried out to 45 to 55° C. to be tempered for 0.5 to 1.5 minutes. Subsequently, the wire rod was heated at 440 to 460° C. and maintained for 2 to 4 minutes at the temperature. Subsequently, the wire rod was subjected to hardening in a soldering bath for rapid cooling. Through the hardening and the tempering, martensite was formed in the wire rod to secure strength. In addition, through hardening in a soldering bath, tempered martensite was formed at a surface of the wire rod to secure strength and toughness.

Next, test examples to confirm properties of the spring steels according to the examples and comparative examples are examined.

The spring steels according to the examples and comparative examples were subjected to tensile strength, yield strength, hardness, fatigue strength, moldability, and fatigue life tests, and a test related to a regulation on inclusions. Results are summarized in FIG. 2.

Here, the yield strength and the tensile strength were measured using specimens having a wire diameter of 3.3 mm by means of a 20 ton tester according to KS B 0802. Hardness was measured under 300 gf loads by means of a micro-Vickers hardness tester according to KS B 0811. Fatigue strength and fatigue life of specimens were measured through a rotary bending fatigue test according to KS B ISO 1143. So as to measure moldability, valve springs having diameter/wire diameter of 6.5 and a turn number of 8 were manufactured, and it was determined as normal when breakage did not occur upon manufacturing of 10,000 valve springs.

To perform a test related to a regulation on the inclusions, each specimen was rolled in parallel, followed by being cut along a centerline and collected. Maximum sizes of B-based and C-based inclusions present in a surface to be tested were measured using a Max. t method with respect to an area of 60 mm². Here, a microscope magnification was 400 to 500 times, and it was determined as being normal when inclusions having a fraction size of greater than 15 μm were not present, the content of inclusions having a fraction size of 10 to 15 μm was 10% or less, and the content of inclusions having a fraction size of 10 μm was 90% or more. Here, the B-based inclusions formed a group in a processing direction and granular inclusions were discontinuously agglomerated and lined up in rows. For example, the B-based inclusions might be alumina (Al₂O₃)-based inclusions, and the C-based inclusions might be silicate (SiO₂)-based inclusions which were not viscously deformed and irregularly dispersed.

As illustrated in FIG. 2, since conventional steels do not include Mo, Ni, V, Nb, and Ti, they pass regulations related to moldability and inclusions, but do not satisfy regulated requirements of the present disclosure related to tensile strength, yield strength, hardness, fatigue strength and fatigue life.

Comparative Examples 1 to 12 do not satisfy alloy ingredient contents regulated in the present disclosure. Comparative Examples 1 to 12 exhibit partially enhanced tensile strength, yield strength, hardness, fatigue strength, moldability, and fatigue life compared to conventional steel, but do not satisfy regulated requirements of the present disclosure.

In particular, since Comparative Example 1 includes a small amount of Mo, yield strength is not sufficiently secured. Thus, tensile strength and yield strength are slightly increased and hardness, fatigue strength, moldability, and fatigue life are decreased, compared to conventional steel.

The contents of Mo, Ni, V, and Ti in each of Comparative Examples 2, 3, 6, 9 and 10 do not satisfy regulated requirements, thus not passing regulations on inclusions. It was confirmed that the inclusions became coarse or non-uniformity of the ingot steel affected formation of the inclusions during a steel-making process and thus it did not pass the regulation on the inclusions.

In addition, it can be confirmed that, in Comparative Example 9 in which the content of Ti is smaller than a regulated requirement, generation of other inclusions such as Al₂O₃ is promoted and thus fatigue durability is decreased, whereby fatigue strength and fatigue life are similar or rather decreased, compared to conventional steel.

On the other hand, all of Examples 1 to 3 which satisfy all regulated requirements of the present disclosure exhibit a tensile strength of about 2300 MPa or more, a yield strength of about 2800 MPa or more and a hardness of about 710 HV or more. In addition, a fatigue strength of about 1100 MPa or more is exhibited, and moldability and regulations on inclusions are passed. Further, a fatigue life of 400,000 times or more is exhibited.

Meanwhile, FIG. 3 illustrates graphs representing calculation results for phase transformation according to temperatures of ultra-high-strength spring steel according to an embodiment of the present disclosure, and FIG. 4 illustrates graphs representing calculation results for phase transformation according to temperatures in cementite tissue of ultra-high-strength spring steel according to an embodiment of the present disclosure.

FIG. 3 illustrates graphs representing calculation results for phase transformation according to temperatures of an example having an alloy composition such as Fe-1.4Si-0.7Mn-0.7Cr-0.55C-0.3Ni-0.1Mo-0.1V. FIG. 3 shows that various carbide types such as CrC and VC, other than FCC-A1 (austenite), BCC-A2 (Ferrite), cementite, etc., are generated when an alloy composition according to the present disclosure is satisfied, and thus, strength increase and fatigue life enhancement may be anticipated.

In addition, FIG. 4 illustrates graphs representing calculation results for phase transformation according to temperatures in cementite tissue of spring steel of an example having an alloy composition such as Fe-1.4Si-0.7Mn-0.7Cr-0.55C-0.3Ni-0.1Mo-0.1V. FIG. 4 shows that it can be predicted that composite behaviors of septenary to octanary elements occur in cementite and thus it may be anticipated that minute carbides are uniformly distributed.

As is apparent from the above description, the present disclosure provides ultra-high-strength spring steel having a high tensile strength of 2300 MPa or more, and a superior fatigue strength of 1100 MPa or more through refinement of inclusions, by optimizing the contents of main alloy ingredients according to an embodiment of the present disclosure.

Although the preferred embodiments of the present disclosure have been disclosed for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the disclosure as disclosed in the accompanying claims. 

What is claimed is:
 1. An ultra-high-strength spring steel, for use as valve spring steel in a vehicle engine, comprises: 0.5 to 0.7% by weight of C, 1.2 to 1.5% by weight of Si, 0.6 to 1.2% by weight of Mn, 0.6 to 1.2% by weight of Cr, 0.1 to 0.5% by weight of Mo, 0.05 to 0.8% by weight of Ni, 0.05 to 0.5% by weight of V, 0.05 to 0.5% by weight of Nb, 0.05 to 0.3% by weight of Ti, 0.3% or less by weight of Cu (but not 0%), 0.0001 to 0.3% by weight of Al, 0.03% or less by weight of N (but not 0%), 0.0001 to 0.003% by weight of O, and a remainder of Fe and other unavoidable impurities, based on 100% by weight of the ultrahigh-strength spring steel.
 2. The ultra-high-strength spring steel according to claim 1, wherein the spring steel has a tensile strength of 2300 MPa or more.
 3. The ultra-high-strength spring steel according to claim 1, wherein the spring steel has a fatigue strength of 1100 MPa or more.
 4. The ultra-high-strength spring steel according to claim 1, wherein the spring steel has a yield strength of 2800 MPa or more.
 5. The ultra-high-strength spring steel according to claim 1, wherein the spring steel has a hardness of 710 HV or more.
 6. The ultra-high-strength spring steel according to claim 1, wherein the sizes of inclusions present in the spring steel are 15 μm or less.
 7. The ultra-high-strength spring steel according to claim 1, wherein, in the inclusions, a fraction of inclusions having sizes of 10 to 15 μm is 10% or less and a fraction of inclusions having sizes of less than 10 μm is 90% or more. 